Solar cell comprising a metal-oxide buffer layer and method of fabrication

ABSTRACT

A perovskite-based solar cell comprising a transparent electrode disposed on a buffer layer that protects the perovskite from damage during the deposition of the electrode is disclosed. The buffer material is deposited using either low-temperature atomic-layer deposition, chemical-vapor deposition, or pulsed chemical-vapor deposition. In some embodiments, the perovskite material is operative as an absorption layer in a multi-cell solar-cell structure. In some embodiments, the perovskite material is operative as an absorption layer in a single-junction solar cell structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This case is a divisional application of co-pending U.S. Non-Provisionalapplication Ser. No. 16/334,540, filed Mar. 19, 2019 (Attorney Docket:146-065US1), which is a national-stage application of PCT ApplicationPCT/US2017/051753, which claims priority of U.S. Provisional PatentApplication Ser. No. 62/397,293, filed Sep. 20, 2016 (Attorney Docket:146-065PR1) and U.S. Provisional Patent Application Ser. No. 62/398,220,filed Sep. 22, 2016 (Attorney Docket: 146-065PR2), each of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under contractDE-EE0006707 awarded by the Department of Energy. The Government hascertain rights in the invention.

If there are any contradictions or inconsistencies in language betweenthis application and one or more of the cases that have beenincorporated by reference that might affect the interpretation of theclaims in this case, the claims in this case should be interpreted to beconsistent with the language in this case.

FIELD OF THE INVENTION

The present invention relates to solar cells in general, and, moreparticularly, to the formation of transparent electrodes for solarcells.

BACKGROUND OF THE INVENTION

A solar cell is an optoelectronic semiconductor device that converts theenergy of incident light directly into electricity. The light isabsorbed in an absorption layer of the solar-cell, which gives rise tothe generation of free electrical carriers (i.e., electrons and holes).The free electrical carriers produce a voltage across the terminals ofthe device, which can be used to directly power electrical systems or bestored in an electrical storage system (e.g., a battery, etc.) for lateruse.

In order to generate a free-carrier pair in the absorption material, aphoton must have energy greater than energy bandgap (EG) of thematerial. The EG of a material is the energy difference between the topof its valence band (the highest energy level populated by boundelectrons) and the bottom of its conduction band (the lowest energylevel populated by free electrons). When a photon is absorbed, itsenergy is given over to a bound electron-hole pair, which frees theelectron and enables it to go from the valence band into the conductionband. The energy of a photon is inversely proportional to its wavelength(E_(p)=hc/λ, where E_(p) is photon energy, h is Planck's constant, c isthe speed of light, and λ is wavelength); therefore, longer-wavelengthlight (e.g., red light) has lower photon energy than shorter-wavelengthlight (e.g., blue light). As a result, the choice of semiconductor usedto absorb the light has significant impact on the efficiency of a solarcell.

Silicon is perhaps the most commonly used solar-cell material atpresent, due to its relatively low EG, highly developed fabricationinfrastructure, and low cost as compared to other semiconductormaterials. Unfortunately, silicon does not efficiently absorb light. Inaddition, since the free electrons and free holes tend to occupy energylevels at the bottom of the conduction band and the top of the valenceband, respectively, any extra energy that the electron-hole pairsreceive from higher-energy photons is lost as heat that is transferredinto the semiconductor material in a process referred to as“thermalization.” Thermalization loss both reduces the energy-conversionefficiency of the solar cell and raises the temperature of the device,which can lead to degradation and lifetime issues.

Both energy-conversion efficiency and thermalization loss can beimproved by combining different materials in a “tandem solar cell”configuration. A tandem solar cell is a stacked structure comprising atop photovoltaic portion that is made of a material having a relativelyhigher EG and a bottom photovoltaic portion that is made of a materialhaving a relatively lower EG. In other words, a tandem solar cell hastwo p-n junctions and two different band gaps. When light is incident onthe solar cell, high-energy photons are first absorbed in the topportion, while lower-energy photons pass through the top portion to beabsorbed in a bottom photovoltaic portion. This enables a broaderspectrum of light to be absorbed, thereby improving energy-conversionefficiency beyond the single-junction efficiency limit. In addition,thermalization loss due to the absorption of high-energy photons in thebottom cell is reduced. Depending on the EG of the material of the topsolar cell, the fundamental efficiency limit for silicon-based tandemsolar cells can be as high as approximately 39%—significantly higherthan the theoretical efficiency limit of 33.7% for a single-junctionsilicon solar cell.

Perovskites are among the most attractive materials for use in bothsingle-junction and tandem-solar-cell structures. In recent years,single-junction perovskite-based solar cell efficiencies have becomeextremely competitive. Their rapid rise is the result of a uniquecombination of properties, such as strong optical absorption and longambipolar diffusion lengths enabled by the benign nature of theintrinsic defects. In addition, perovskite-based solar cells have wide,tunable bandgaps and solution processability, which makes themparticularly attractive for use as the top cell in tandem solar cellconfigurations having bottom cells comprising a lower-EG materials, suchas silicon, copper indium gallium selenide (CIGS), etc. They present,therefore, a pathway to achieving industry goals of improving efficiencywhile continuing to drive down module cost.

Perovskite-based tandem solar cells have been demonstrated in bothmechanically stacked, four-terminal configurations and monolithicallyintegrated two and three-terminal configurations. Mechanically stackedtandem structures have seen the largest success, recently reaching apower conversion efficiency over 24%, as the architecture simplifiesdevice fabrication, allows for silicon surface texturing, and requiresno current matching. Monolithically integrated tandem structures,however, have greater promise due to the fact that they have fewersemitransparent electrode layers.

To date, however, the commercial viability of both single-junction andtandem perovskite-based solar cells has been limited due to thermal andenvironmental instability. In addition, the efficiency of monolithicperovskite-based tandem solar cells continues to lag behind theirmechanically stacked counterparts, largely due to difficulties indepositing a transparent electrical contact on the perovskite-based topcell, where the transparent electrical contact is suitable as a highlytransmissive window layer. Due to fabrication constraints, light mustfirst pass through a hole-transport layer (e.g., Spiro-OMeTAD) in astandard architecture, or an electron-acceptor layer (e.g.,[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)) in an invertedarchitecture, before entering the perovskite, which gives rise tosignificant parasitic losses. In addition, in order to deposit a highlytransparent and conductive electrode (e.g., indium tin oxide (ITO),etc.), a buffer layer is required to protect the perovskite and organiccarrier-extraction layers from damage due to the high energy ofsputtered particles. Prior-art buffer layers, however, have poorlong-term stability due to their chemical reactivity with perovskitecompositions. Further, prior-art buffer layers have been plagued by lowefficiency, which degrades fill factor and open-circuit voltage.

The need for a high-quality, highly transparent electrical contactsuitable for use as a window layer in a solar cell and that is readilyformed on a perovskite solar cell structure remains, as yet, unmet inthe prior art.

SUMMARY OF THE INVENTION

The present invention enables high-conversion-efficiencyperovskite-based solar cells that are environmentally stable andcommercially viable. Embodiments of the present invention employ abuffer layer disposed on the surface of a perovskite-based absorptionlayer to protect it during the formation of an electrical contact on itssurface, where the buffer layers are substantially non-reactive withperovskite materials, are energetically well aligned to act as a carrierextraction layer, and are thin enough to mitigate optical absorption.Embodiments of the present invention are particularly well suited forsingle-cell and multi-cell solar-cell structures (e.g., tandem solarcells, etc.).

An illustrative embodiment of the present invention is a single-junctionsolar cell disposed on a glass substrate, wherein the solar cellcomprises a perovskite-based absorption layer (i.e., cell), a lowerelectrical contact of ITO, and a substantially transparent upperelectric contact that functions as a window layer for the solar cell.The upper electrical contact includes an ITO layer sputter deposited ona buffer that comprises a bilayer of metal oxides. The metal oxidelayers are deposited on the perovskite absorption layer vialow-temperature atomic-layer deposition. In some embodiments, the bufferlayer is deposited using pulsed-chemical vapor deposition.

In some embodiments, a perovskite layer is included as one of multipleabsorption layers in a multi-cell solar-cell structure, such as a tandemsolar cell. In some embodiments, a perovskite layer is included as thetop absorption layer in a tandem solar cell having a bottom cellcomprising silicon. In some embodiments, a perovskite layer is includedas the top absorption layer in a tandem solar cell having a bottom cellcomprising a material other than silicon, such as CIGS, a secondperovskite that is characterized by a lower EG than the perovskite ofthe top cell, and the like.

In some embodiments, a solar-cell structure includes a buffer layeroperative as a barrier layer that increases the environmental stabilityof the perovskite.

In some embodiments, a solar-cell structure includes a buffer layerhaving only one layer of metal oxide. In some of these embodiments, thebuffer consists of tin oxide.

In some embodiments, a buffer layer is deposited using chemical-vapordeposition. In some embodiments, a buffer layer is deposited usingpulsed chemical-vapor deposition.

In some embodiments, a buffer layer is provided such that it issubstantially non-reactive with halides.

In some embodiments, a solar-cell structure includes a buffer layer thatis operative as a carrier extraction layer in the solar cell.

An embodiment of the present invention is a solar cell (100) that isoperative for converting a first light signal (124) into electricalenergy, the solar cell including: a first absorption layer (110)comprising a first material (112), the first material being a perovskiteand having a first energy bandgap; an electrical contact (122) that isin electrical communication with the first absorption layer, theelectrical contact being substantially transparent for a firstwavelength range, the first light signal including light within thefirst wavelength range; and a buffer layer (116) that is between theelectrical contact and the first absorption layer, the buffer layercomprising a first layer (118) comprising a first metal oxide.

Another embodiment of the present invention is a method for forming aperovskite-based solar cell operative for converting a first lightsignal (124) into electrical energy, the method including: providing afirst absorption layer (110) comprising a first perovskite (112), thefirst absorption layer being operative for absorbing at least a firstportion (P1) of the first light signal; forming a buffer layer (116)that is disposed on the first absorption layer, the buffer layerincluding a first layer (118) comprising a first metal oxide, whereinthe first layer is formed at a temperature that is less than or equal to150° C.; and forming a first electrical contact (122) on the bufferlayer, wherein the first electrical contact is substantially transparentfor at least a portion of the first light signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of a cross-sectional view of aportion of a solar cell in accordance with an illustrative embodiment ofthe present invention.

FIG. 2 depicts operations of a method suitable for forming asingle-junction perovskite-based solar cell in accordance with theillustrative embodiment of the present invention.

FIG. 3 depicts measurement data for solar cells having differentelectrodes.

FIG. 4 depicts a plot of the maximum power output for a single-junctionsolar cell in accordance with the present invention.

FIG. 5 depicts measurement data of external quantum efficiency (EQE),transmission, and reflection for a single-junction solar cell inaccordance with the present invention.

FIG. 6 depicts a schematic drawing of a cross-sectional view of a tandemsolar cell in accordance with the present invention.

FIG. 7 depicts operations of a method suitable for forming amulti-junction solar cell in accordance with the present invention.

FIGS. 8A-B depict measured performance data for a tandem solar cell inaccordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a schematic diagram of a cross-sectional view of aportion of a solar cell in accordance with an illustrative embodiment ofthe present invention. Solar cell 100 comprises substrate 102, bottomcontact 104, perovskite cell 110, electron-accepter layer 114, bufferlayer 116, and contact 122. Solar cell 100 generates output voltage, V1,when illuminated with light signal 124 (i.e., solar radiation). Solarcell 100 is a perovskite-based single-junction solar cell that has aninverted architecture (i.e., the perovskite material is disposed on topof the p-type selective contact).

FIG. 2 depicts operations of a method suitable for forming asingle-junction perovskite-based solar cell in accordance with theillustrative embodiment of the present invention. Method 200 isdescribed herein with continuing reference to FIG. 1. Method 200 beginswith operation 201, wherein bottom contact 104 is formed on substrate102.

Substrate 102 is a conventional glass substrate suitable forplanar-processing operations. Preferably, substrate 102 comprises amaterial that is transparent for solar radiation, such as glass;however, other materials can be used for substrate 102 without departingfrom the scope of the present invention. Materials suitable for use insubstrate 102 include, without limitation, polymers, glasses,semiconductors, ceramics, composite materials, and the like.

Bottom contact 104 is a two-layer structure that includes transparentconductive layer 106 and hole-selective contact layer 108.

Transparent conductive layer 106 is formed on substrate 102 inconventional fashion and includes an electrically conductive materialthat is substantially transparent for solar radiation. In the depictedexample, transparent conductive layer 106 comprises ITO; however, insome embodiments, it includes a different transparent conductivematerial, such as zinc tin oxide (ZTO), indium zinc oxide (IZO),titanium nitride, tantalum nitride, electrically conductive nanoscalewires, and the like.

Hole-selective contact layer 108 (hereinafter referred to as layer 108)comprises nickel oxide (NiO). In some embodiments, hole-selectivecontact layer 108 includes a different hole-selective material, such aspoly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS21),poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), and the like. Inthe depicted example, layer 108 is formed on transparent conductivelayer 106 by spinning a 1M solution of nickel nitrate andethylenediamine in ethylene glycol on layer 106 and annealing thematerial at 300° C.

At operation 202, perovskite cell 110 is formed on bottom contact 104.Perovskite cell 110 comprises material 112 and functions as theabsorption layer of solar cell 100. Perovskite cell 110 is formed bydispensing a stoichiometric solution containing cesium iodide (CsI),formamidinium iodide (FAI), lead iodide (PbI₂), and lead bromide (PbBr₂)in a mixture of Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO)and spinning the substrate to yield a substantially uniform coating of(Cs_(0.17)FA_(0.83)Pb(Br_(0.17)I_(0.83))₃ perovskite. During spincoating, chlorobenzene is used as an antisolvent to assist perovskitecrystallization. After it is formed, the coating is annealed on a hotplate at 50° C. for 1 minute and then at 100° C. for 30 minutes.

A perovskite cell containing (Cs_(0.17)FA_(0.83)Pb(Br_(0.17)I_(0.83))₃perovskite is particularly desirable in embodiments of the presentinvention because it is more thermally stable than many otherperovskites. Its use, therefore, affords embodiments of the presentinvention particular advantages. Furthermore, mixed-cation-perovskitesolar cells have consistently outperformed their single-cationcounterparts based on published literature. In fact, the firstperovskite device to break 20% was fabricated with a mixture ofmethylammonium (MA) and formamidinium (FA). In addition, the inclusionof cesium enables high conversion efficiency with improved photo,moisture, and thermal stability. Improved moisture and thermal stabilityare particularly advantageous because they provide greater processingfreedom when forming additional structures on top of the perovskitematerial. It should be noted, however, that myriad perovskites (e.g.,pure methylammonium based perovskite (MAPbI₃), etc.) suitable for use incell 110 exist and can be used without departing from the scope of thepresent invention.

At optional operation 203, electron-acceptor layer 114 is formed onperovskite cell 110. The formation of electron-acceptor layer 114 beginswith the thermally evaporation of a layer of lithium fluoride (LiF)having a thickness of approximately 1 nm. The LiF layer acts as apassivation layer. This is followed by the thermal evaporation of alayer of PCBM having a thickness of approximately 10 nm. The thin layerof PCBM enables good electron-extraction properties, while stillachieving high optical transmission. In some embodiments, PCBM alsoprevents the development of an extraction barrier. Although theillustrative embodiment includes an electron-acceptor layer comprisingPCBM, it will be clear to one skilled in the art, after reading thisSpecification, how to specify, make, and use alternative embodimentsthat include an electron-acceptor layer comprising a different material,such as C₆₀-based triads, and the like.

At operation 204, buffer layer 116 is formed on electron acceptor-layer114 such that the buffer layer is conformal with surface S1 ofperovskite cell 110 (including electron-acceptor layer 114). Bufferlayer 116 is formed such that it is substantially transparent for solarradiation, preferably has little or no parasitic absorption, and iscapable of efficient electron extraction.

It is an aspect of the present invention that forming a thin conformalbuffer layer using a deposition technique that does not damage sensitivelayers beneath it affords significant advantages over the prior art.Specifically, a thin conformal layer avoids the need for thickplanarizing layers used in prior-art solar cell structures, which aretypically necessary in the prior art to prevent shunt pathways but whichcan inhibit transmission of solar radiation. Although other depositionmethods can be used without departing from the scope of the presentinvention, preferably buffer layer 116 is deposited using one ofatomic-layer deposition (ALD), chemical-vapor deposition (CVD), andpulsed CVD (p-CVD).

Inset1 of FIG. 1 is a scanning-electron microscope (SEM) image thatshows the conformal nature of an ALD-deposited buffer layer on thesurface of a perovskite cell in accordance with the present invention.

In the depicted example, buffer layer 116 is a bilayer comprising layers118 and 120, which comprise tin oxide (SnO₂) and zinc tin oxide (ZTO),respectively.

Layer 118 is a layer of tin oxide having a thickness of approximately 4nm. It is formed via ALD deposition using tetrakis(dimethylamino)tin(IV)(TDMASn) and water at approximately 30° C. In some embodiments, layer118 is formed via ALD deposition using a different precursor and/or at adifferent temperature; however, it is preferable that the deposition bedone at temperatures at or below 150° C. Low-temperature deposition isdesirable because, although tin-oxide films have been shown to operateas effective electron-selective contacts, high deposition temperaturesare known to degrade the stoichiometry and electronic properties ofperovskite films.

An exemplary process for depositing layer 118 includes a 1.5-second longpulse of Tetrakis(dimethylamino)tin(IV) (TDMA-Sn) precursor, followed bya 30 second purge, a 1-second long deionized water pulse, and a second30-second long purge, which yields a growth rate of 1.2 Å/cycle. Duringthe deposition process, the door and body temperature of the ALD reactor(an Arradiance GEMStar-6 ALD system) are maintained at 100° C., whilethe manifold temperature is maintained at 115° C. The temperature of theprecursor is held at 60° C. and each purge is conducted with a 90 sccmnitrogen flow rate.

After deposition of the tin-oxide layer, buffer layer 116 is completedwith the formation layer 120, which is a layer of ZTO having a thicknessof approximately 2 nm. Layer 120 is included in buffer layer 116 tofacilitate good electrical connection with contact 122. However, asdiscussed below, the inclusion of layer 120 in buffer 116 is optional.

In the depicted example, layer 120 is formed using a combination ofalternating tin-oxide depositions zinc oxide depositions, where the twoprocesses are repeated in a “supercycle” comprising three cycles oftin-oxide deposition and three cycles of zinc-oxide deposition.

An exemplary process for depositing layer 120 includes a 100-millisecondlong pulse of diethyl zinc (DEZ) precursor and water, followed by a 30second purge, a 1-second long deionized water pulse, and a second30-second long purge. This process gives rise to a growth rate of 1.8Å/cycle. As in the deposition process described for layer 118 above,during the zinc-oxide deposition process, the door and body temperatureof the ALD reactor (an Arradiance GEMStar-6 ALD system) are maintainedat 100° C., while the manifold temperature is maintained at 115° C. andeach purge is conducted with a 90 sccm nitrogen flow rate; however,during the deposition layer 120, the DEZ precursor is unheated. Thisexemplary process yields an effective growth rate of approximately 5.8Å/supercycle.

It should be noted that tin oxide, by itself, is a good electronextractor, is compatible with perovskite materials, and can survive awide range of deposition techniques, such as sputtering, CVD,evaporation, physical vapor deposition, spin coating, etc., that mightbe desirable for subsequent processing operations in the fabrication ofsolar cell 100 (e.g., the formation of contact 122). As a result, theinclusion of a ZTO layer in buffer layer 116 is optional. In someembodiments, therefore, buffer layer 116 is a single-layer structurethat includes only tin oxide.

It should also be noted that tin oxide and zinc tin oxide are only twoexamples of materials suitable for use in buffer layer 116. In someembodiments, therefore, buffer layer 116 is a single-layer ormulti-layer structure that includes one or more layers of anotherelectron- or hole-transport material. Alternative materials suitable foruse as buffer layer materials in embodiments of the present inventioninclude, without limitation, titanium dioxide, nickel oxide, platinumoxide, vanadium oxide, tungsten oxide, molybdenum oxide and the like.

As noted above, each of layers 118 and 120 is deposited using ALD. ALDis preferred for depositing the buffer layer materials because it is avapor-phase deposition technique capable of depositing conformal thinfilms through sequential self-limiting surface reactions. The use of ALDfor the deposition of buffer layer materials affords some embodiments ofthe present invention with particular advantage over other prior-artdeposition methods because ALD enables the fabrication of uniform,conformal, thin films having high optical transmission, regardless ofthe texture of the surface on which it is deposited. A wide range ofmetals and metal oxides can be deposited by ALD for photovoltaicapplications. As a result, embodiments of the present invention mitigateproblems associated with the typical high surface roughness of manyperovskite materials, such as cesium formamidinium (CsFA) perovskites.In addition, many prior-art deposition methods, such as spin coating,require very thick layers to be formed to planarize the surface andprevent shunt pathways, which would lead to poor transmission. Inaddition, ALD is a soft-deposition method that typically does not damageorganic extraction layers and forms compact metal-oxide layers that actas effective buffer layers during the subsequent sputter deposition ofITO to form a top electrical contact for the solar cell (i.e., contact122).

Although ALD is the preferred method for depositing the materials ofbuffer layer 116, in some alternative embodiments, at least one layer ofbuffer layer 116 is formed using CVD or p-CVD.

CVD is a deposition process wherein one or more gaseous or plasmaprecursors flow and react or decompose on or near a surface in acontinuous fashion. Surface reaction or decomposition is preferred toprevent nucleation above the surface, which would degrade the quality ofthe resultant layer and is often achieved through thermal degradation ofprecursor through high substrate temperatures. In embodiments of thepresent invention, CVD-deposition is performed using highly reactiveprecursors that decompose at low temperature to avoid degradation of theperovskite material. In addition, low-temperature CVD depositionprovides more conformal films, which enables buffer layer 116 to be thinyet outperform prior-art layers, such as spun metal-oxide nanoparticlesor organic materials. In addition, it enables deposition of materialsthat cannot be easily evaporated.

p-CVD is a modified ALD process having a continual, rather than purelystep-wise, growth component. By separating the precursor pulses in thep-CVD regime, the benefits of surface growth and high uniformityprovided by ALD are achieved; however, growth rates can be higher due tolingering precursor that gives rise to a CVD growth mode, whichmitigates perovskite degradation and enables faster processing. Thetotal process time required for forming ALD buffer layer 116 is reducedby reducing purge times from 30 sec to 5 sec, thereby leaving the “trueALD” regime and entering the pulsed-CVD growth regime. Using p-CVD, agrowth rate of 0.5 nm/min can be achieved, resulting in a total contactdeposition time of approximately 15 min. It should be noted that thestoichiometry and performance of the deposited SnO₂ and ZTO layersthrough ALD and pulsed-CVD are substantially identical. This isparticularly important for some embodiments of the present inventionsince CVD is a well-known commercially viable and scalable process, andis already used in typical silicon solar-cell fabrication—for example,to deposit SiN_(x) anti-reflection coatings for diffused-junction cellsor amorphous silicon passivation layers for silicon heterojunction solarcells.

At operation 205, contact 122 is formed on buffer layer 116 to provide atop electrical contact that is substantially transparent for theradiation of light signal 124 (i.e., the “window layer” of solar cell100). Contact 122 is formed via sputter deposition of an ITO layerhaving a thickness of approximately 400 nm. In some embodiments, contact122 includes metal (e.g. silver, etc.) features, such as, traces,fingers, etc. and/or an antireflective coating (e.g. LiF, etc.). It isanother aspect of the present invention that the presence of the robustbuffer layer 116 on the top of the perovskite cell structure enables thesputter deposition of the ITO as a transparent electrode with little orno damage to the underlying PCBM layer (i.e., electron-acceptor layer114) or perovskite cell 110.

Although sputter deposition of ITO is preferred for forming contact 122,in some embodiments, a different transparent conductive material is usedin contact 122. Furthermore, the inclusion of buffer layer 116 enablesthe use of other deposition methods for forming contact 122 withoutdeparting from the scope of the present invention. Materials suitablefor use in contact 122 include, without limitation, transparentconductive oxides (e.g., indium tin oxide, zinc tin oxide, indium zincoxide, etc.), diffusion barriers (e.g., titanium nitride, tantalumnitride, etc.), metals (e.g., silver, gold, etc.), metal nanowires, andthe like. Deposition processes suitable for use in depositing thematerial of contact 122 include, without limitation, sputter deposition,thermal evaporation, e-beam evaporation, chemical vapor deposition,physical vapor deposition, spin coating, and the like.

It should be noted that the materials and layer thicknesses providedherein are merely exemplary and that alternative materials and/or layerthicknesses can be used without departing from the scope of the presentinvention.

FIG. 3 depicts measurement data for solar cells having differentelectrodes. Plot 300 includes traces 302 and 304, where trace 302 is theJ-V curve of a semi-transparent device in accordance with the presentinvention and trace 304 is the J-V curve for a comparable devicestructure having an opaque top electrical contact comprising aluminum.Each device includes a buffer layer comprising ALD-deposited SnO₂ andZTO layers. A careful comparison of traces 302 and 304 reveals that theinclusion of buffer layer 116 in the solar cell structure makes itpossible to sputter deposit an ITO-based contact 122 that issubstantially transparent while also remaining highly effective as anelectrical contact.

FIG. 4 depicts a plot of the maximum power output for a single-junctionsolar cell in accordance with the present invention. Plot 400 shows astable power output at 14.5% for a semi-transparent perovskite device inaccordance with the present invention

FIG. 5 depicts measurement data of external quantum efficiency (EQE),transmission, and reflection for a single-junction solar cell inaccordance with the present invention. Plot 500 shows the externalquantum efficiency (EQE), transmission, and reflection through substrate102 (traces 502, 504, and 506, respectively) and through contact 122(traces 508, 510, and 512, respectively). Plot 500 evinces the enhancedoptical properties of the ALD SnO₂, since the EQE is substantially equalthrough either side of the device, while the transmission at longerwavelengths remains high and averages around 70% through contact 122.

FIG. 6 depicts a schematic drawing of a cross-sectional view of a tandemsolar cell in accordance with the present invention. Solar cell 602 is asilicon heterojunction solar cell, which has a high open-circuit voltageresulting from the separation of highly recombination-active (ohmic)contacts from the silicon absorber bulk.

Solar cell 600 includes solar cell 100 and solar cell 602, which aremonolithically integrated on substrate 604.

Solar cell 100 and solar cell 602 are arranged such that light signal124 is first incident the absorption layer of solar cell 100, whichabsorbs portion P1 of the solar radiation while second portion P2 passesthrough it to the absorption layer of solar cell 602. Portion P1includes light having bandgap equal to or greater than the energybandgap of the absorption layer of solar cell 100, while portion P2includes light having energy less than the energy bandgap of theabsorption layer of solar cell 602 (and, typically, some unabsorbedlight of portion P1).

Solar cell 100 and solar cell 602 are monolithically integrated onsubstrate 604 to form a monolithically integrated tandem solar cellstructure. For the purposes of this Specification, including theappended claims, the term “monolithically integrated” is defined asformed by depositing layers on a substrate via one or more thin-filmdeposition processes and, optionally, patterning the deposited layersafter deposition. The term monolithically integrated explicitly excludesstructures wherein two or more fully fabricated devices are joined,after their fabrication, to form a unitary structure.

FIG. 7 depicts operations of a method suitable for forming amulti-junction solar cell in accordance with the present invention.Method 700 is described herein with continuing reference to FIG. 6.Method 700 begins with operation 701, wherein surface 608 of substrate604 is textured.

Substrate 604 is a conventional substrate that is suitable for use in aplanar processing fabrication sequence. In the depicted example,substrate 604 comprises material 630 and functions as the bottom cell(i.e., bottom absorption layer) in tandem solar cell 600. In thedepicted example, substrate 604 is an n-type, 280-μm-thick, double-sidepolished float-zone (FZ) silicon wafer (i.e., material 630 issingle-crystal silicon); however, other substrates can be used withoutdeparting from the scope of the present invention.

Surface 608 is textured by first depositing a 260-nm-thick,low-refractive-index silicon nitride layer by plasma-enhancedchemical-vapor deposition (PECVD) on surface 606. The substrate is thenimmersed in a potassium hydroxide (KOH) solution, which creates thedesired texture on uncoated surface 608, while surface 606 is protectedby its silicon nitride coating.

After removing the substrate from the KOH solution, the nitride coatingis then removed from surface 606 using diluted hydrofluoric acid (HF),and the wafer is cleaned by immersion in a conventional piranha etchfollowed by a standard RCA-B cleaning sequence.

At operation 702, intrinsic and p-type amorphous silicon (a-Si:H) films614 and 616 are formed on surface 608. These layers are included toprovide surface passivation and facilitate electrical contact betweencontact layer 618 and bottom cell 622. In the depicted example, films614 and 616 are formed with thicknesses of 7 and 15 nm, respectively.Typically, native oxide is removed from the deposition surface usingbuffered oxide etch (BOE) prior to the formation of these layers.

At operation 703, intrinsic and n-type a-Si:H films 610 and 612 areformed on surface 606 to provide surface passivation and facilitateelectrical contact between the transparent conductive layer 626 andbottom cell 622. In the depicted example, films 610 and 612 are formedwith thicknesses of 7 and 8 nm, respectively.

At operation 704, transparent conductive layer 626 is formed on layer612 to complete center contact 624. In the depicted example, transparentconductive layer 626 is a layer of ITO having a thickness ofapproximately 20 nm; however, other materials and thicknesses can beused for transparent conductive layer 626 without departing from thescope of the present invention.

At operation 705, contact layer 618 is formed on a-Si layer 616, whichcompletes bottom contact 620. In the depicted example, contact layer 618comprises a layer of ITO having a thickness of 20 nm; however, othermaterials and thicknesses can be used for contact layer 618 withoutdeparting from the scope of the present invention.

In some embodiments, contact layer 618 includes a layer of siliconnanoparticles a thin layer of silver, where the layer of siliconnanoparticles includes a plurality of openings that enable ohmic contactbetween the silver and ITO layers, thereby facilitating chargecollection at bottom contact 620. The addition of a silicon nanoparticlelayer in contact layer 618 enhances the reflection of lower-energyphotons not absorbed by substrate 604.

Once the bottom cell of tandem solar cell 600 (i.e., silicon solar cell602) is complete, fabrication of the rest of the tandem solar-cellstructure substantially follows the operations of method 200, asdescribed above, with slight deviations to avoid degradation of thematerials included in the bottom cell.

At operation 706, hole-selective layer 628 is formed ontransparent-conductive layer 626 in analogous fashion to the formationof hole-selective layer 108 described above and with respect to method200. However, it should be noted that, in operation 706, hole-selectivecontact layer 628 is annealed at a lower temperature than used to annealhole-selective contact layer 108 to mitigate damage to previously formedlayers in the solar cell structure. In the depicted example,hole-selective contact layer 628 is annealed at only 200° C. rather thanat 300° C. as described above in method 200. One skilled in the art willrecognize, after reading this Specification, that a lower-temperatureanneal must typically have a longer duration; therefore, the annealingstep of operation 706 is normally conducted for 10 hours or more. Insome embodiments, hole-selective layer 628 is annealed at a differenttemperature and/or for a different amount of time.

At operation 707, top cell 610 is formed on hole-selective layer 628.Top cell 610 is analogous to perovskite cell 110 described above.

At operation 708, electron-acceptor layer 114 is optionally formed ontop cell 610.

At operation 709, buffer layer 116 is formed on electron-acceptor layer114.

At operation 710, contact 122 is formed on buffer layer 116 to completethe formation of tandem solar cell 600.

Although solar cell 600 is a tandem solar cell, it will be clear to oneskilled in the art, after reading this Specification, how to specify,make, and use alternative embodiments that are multi-cell solar cellshaving any practical number of junctions.

FIGS. 8A-B depict measured performance data for a tandem solar cell inaccordance with the present invention. The data shown in plots 800 and802 was measured using a solar cell structure analogous to that of solarcell 600 having an active area of 1 cm² and a fill factor ofapproximately 0.79.

Plot 800 depicts a measured J-V scan that shows an open circuit voltage(Voc) of 1.66V and a high short-circuit current density of 18.6 mA/cm²,resulting in an efficiency of 23.7%. In addition, the device maintainedthis efficiency while being held at its maximum power point for morethan one hour under constant illumination. These results demonstratethat the ALD- and p-CVD-deposited buffer layers enable deposition of awindow layer that prevents pinholes and shunt pathways.

Furthermore, the total absorption (1-reflectance) and external quantumefficiencies (EQE) for each sub-cell in the tandem solar-cell structurewere determined as Top cell 100 generates a short circuit currentdensity (JSC) of 18.9 mA/cm2. Silicon solar cell 602 starts respondingfrom 640 nm as benefited from the semi-transparent perovskite top cell,and produces a JSC of 18.5 mA/cm², which is, however, lower than the topcell.

Plot 802 also includes results of a current-loss analysis that revealsthat the surface reflection loss is 4.8 mA/cm². Parasitic absorptionloss is indicated by the gap between 1-reflection and EQE curves,revealing total losses from parasitic absorption at 4.5 mA/cm². Totaldevice 600 current density will be limited by the half cell (i.e. solarcell 100 or silicon solar cell 602) with the lower current density.

Contact 122 acts as a highly transparent and conductive electrode. Inaddition, ITO has good moisture barrier properties that significantlyincrease the thermal and environmental stability of a perovskite solarcell (e.g., solar cell 100) by substantially trapping volatilemethylammonium cations. The combination of the increased thermal andmoisture stability of the mixed formamidinium and cesium perovskite (ascompared to the pure methylammonium perovskite and dense, pinhole freeALD SnO₂ layers in accordance with the present invention affordembodiments of the present invention with improved stability than forprior-art perovskite-based solar cells (either single junction or tandemconfigurations).

This improved stability has been shown by operating CsFA perovskitesolar cells capped with ALD-deposited SnO₂ and ITO at their maximumpower point in ambient atmosphere without additional encapsulation underone-sun equivalent visible illumination with a sulfur plasma lamp. Incontrast to prior-art perovskite-based solar cells, the devices inaccordance with the present invention operated for 1000 hours at orhigher than the initial efficiency. In addition, it is known that smalldust particles in perovskite material can give rise to pinholes in theITO encapsulation, which leads to pathways for methylammonium evolutionand the eventual degradation of power efficiency. CsFA-perovskite-basedsolar cells comprising ALD-deposited SnO₂, however, exhibited nopinholes after 600 hours of operation, thereby evincing the efficacy ofthe conformal ALD process to prevent pinhole formation and to theoverall increased stability of the CsFA perovskite.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

1. A solar cell (100) that is operative for converting a first lightsignal (124) into electrical energy, the solar cell including: a firstabsorption layer (110) comprising a first material (112), the firstmaterial being a perovskite and having a first energy bandgap; anelectrical contact (122) that is in electrical communication with thefirst absorption layer, the electrical contact being substantiallytransparent for a first wavelength range, the first light signalincluding light within the first wavelength range; and a buffer layer(116) that is between the electrical contact and the first absorptionlayer, the buffer layer comprising a first layer (118) comprising afirst metal oxide; wherein the buffer layer is produced by a processselected from the group consisting of atomic-layer deposition (ALE),chemical-vapor deposition (CVD) and pulsed-chemical-vapor deposition(p-CVD), and wherein the buffer layer is formed at a temperature that isless than or equal to 150° C.
 2. The solar cell of claim 1 wherein thefirst metal oxide is tin oxide.
 3. The solar cell of claim 1 wherein thefirst metal oxide is selected from the group consisting of zinc oxide,zinc tin oxide, titanium oxide, nickel oxide, platinum oxide, tungstenoxide, and vanadium oxide.
 4. The solar cell of claim 1 wherein thebuffer layer further includes a second layer (120) comprising a secondmetal oxide.
 5. The solar cell of claim 4 wherein first metal oxide istin oxide and the second metal oxide is zinc tin oxide.
 6. The solarcell of claim 1 wherein the buffer layer is operative for extractingelectrons from the first absorption layer.
 7. The solar cell of claim 1wherein the buffer layer is substantially non-reactive with halides. 8.The solar cell of claim 1 wherein the buffer layer is substantiallyconformal with a first surface (S1) of the first absorption layer. 9.The solar cell of claim 1 wherein the first material comprises cesium.10. The solar cell of claim 1 further comprising a second absorptionlayer (602) comprising a second material (630) having a second energybandgap that is lower than the first energy bandgap, wherein the firstabsorption layer and second absorption layer are monolithicallyintegrated and collectively define a monolithically integrated tandemsolar cell (600).
 11. The solar cell of claim 10 wherein the secondmaterial is selected from the group consisting of silicon, galliumarsenide (GaAs), cadmium telluride (CdTe), copper indium galliumselenide (CIGS), and perovskites.
 12. The solar cell of claim 10 whereinthe first material comprises cesium and the second material is silicon.13-20. (canceled)
 21. A solar cell (100) that is operative forconverting a first light signal (124) into electrical energy, the solarcell including: a first absorption layer (110) comprising a firstmaterial (112), the first material being a perovskite and having a firstenergy bandgap; an electrical contact (122) that is in electricalcommunication with the first absorption layer, the electrical contactbeing substantially transparent for a first wavelength range, the firstlight signal including light within the first wavelength range; and abuffer layer (116) that is between the electrical contact and the firstabsorption layer, wherein the buffer layer consists of metal oxide. 22.The solar cell of claim 21 wherein the buffer layer includes a firstmetal oxide that is selected from the group consisting of tin oxide,zinc oxide, zinc tin oxide, titanium oxide, nickel oxide, platinumoxide, tungsten oxide, and vanadium oxide.
 23. The solar cell of claim21 wherein buffer layer includes a first layer (118) comprising a firstmetal oxide.
 24. The solar cell of claim 23 wherein the buffer layerincludes a second layer (120) comprising a second metal oxide.
 25. Thesolar cell of claim 24 wherein the first metal oxide is tin oxide andthe second metal oxide is zinc tin oxide.
 26. The solar cell of claim 21wherein the buffer layer is operative for extracting electrons from thefirst absorption layer.
 27. The solar cell of claim 21 wherein thebuffer layer is substantially non-reactive with halides.
 28. The solarcell of claim 21 wherein the buffer layer is substantially conformalwith a first surface (S1) of the first absorption layer.
 29. The solarcell of claim 21 wherein the first material comprises cesium.
 30. Thesolar cell of claim 21 further comprising a second absorption layer(602) comprising a second material (630) having a second energy bandgapthat is lower than the first energy bandgap, wherein the firstabsorption layer and second absorption layer are monolithicallyintegrated and collectively define a monolithically integrated tandemsolar cell (600).
 31. The solar cell of claim 30 wherein the secondmaterial is selected from the group consisting of silicon, galliumarsenide (GaAs), cadmium telluride (CdTe), copper indium galliumselenide (CIGS), and perovskites.
 32. The solar cell of claim 30 whereinthe first material comprises cesium and the second material is silicon.